Process for the conversion of lower alkanes to aromatic hydrocarbons

ABSTRACT

A process for producing aromatic hydrocarbons which comprises (a) contacting one or more lower alkanes with a dehyroaromatization aromatic catalyst which is comprised of 0.005 to 0.1% wt platinum, not more than 0.2% wt of an amount of an attenuating metal wherein the amount of platinum is not more than about 0.02% wt more than the amount of the attenuating metal, from about 10 to about 99.9% wt of an aluminosilicate, and a binder, and (b) separating methane, hydrogen, and C 2-5  hydrocarbons from the reaction products of step (a) to produce aromatic reaction products including benzene.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 12/608671,filed Oct. 29, 2009, which is a continuation-in-part of U.S. NonProvisional Application Ser. No. 12/371787, filed Feb. 16, 2009, whichclaims priority to U.S. Provisional Application Ser. No. 61/029481 filedFeb. 18, 2008, the entire disclosures of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to a process for producing aromatichydrocarbons from lower alkanes. More specifically, the inventionrelates to a dehydroaromatization process for increasing the productionof benzene and/or total aromatics from lower alkanes.

BACKGROUND OF THE INVENTION

There is a projected global shortage for benzene which is needed in themanufacture of key petrochemicals such as styrene, phenol, nylon andpolyurethanes, among others. Generally, benzene and other aromatichydrocarbons are obtained by separating a feedstock fraction which isrich in aromatic compounds, such as reformates produced through acatalytic reforming process and pyrolysis gasolines produced through anaphtha cracking process, from non-aromatic hydrocarbons using a solventextraction process.

In an effort to meet growing world demand for benzene and otheraromatics, various industrial and academic researchers have been workingfor several decades to develop catalysts and processes to make lightaromatics (benzene, toluene, xylenes, or BTX) from cost-advantaged,light paraffin (C₁-C₄) feeds. Prior-art catalysts devised for thisapplication usually contain an acidic zeolite material such as ZSM-5 andone or more metals such as Pt, Ga, Zn, Mo, etc. to provide adehydrogenation function. Aromatization of ethane and other loweralkanes is thermodynamically favored at high temperature and lowpressure without addition of hydrogen to the feed. Unfortunately, theseprocess conditions are also favorable for rapid catalyst deactivationdue to formation of undesirable surface coke deposits which block accessto the active sites.

For many hydrocarbon processing applications, one approach to reducingcatalyst performance decline rates due to coking is to increase thecatalyst metals loading in an effort to promote fasterhydrogenation/breakup of large coke precursor molecules on the surface.Another approach involves incorporation of additives such as phosphateor rare earths to moderate surface acidity and reduce coking rates underreaction conditions. These approaches are appropriate for processesfeaturing fixed or slowly-moving catalyst beds wherein the averagecatalyst particle residence time in the reactor zone betweenregenerations (coke burnoff steps) is relatively long (at least severaldays). For example, see U.S. Pat. Nos. 4,855,522 and 5,026,937, whichdescribe ZSM-5-type lower-alkane aromatization catalysts promoted withGa and additionally containing either a rare earth metal or aphosporus-containing alumina, respectively.

Yet another approach to circumvent this problem is to devise a loweralkane aromatization process in which the catalyst spends a relativelyshort time (less than a day) under reaction conditions before beingsubjected to coke burnoff and/or other treatment(s) aimed at restoringall or some of the original catalytic activity. An example of such aprocess is one featuring two or more parallel reactors containing fixedor stationary catalyst beds, with at least one reactor offline forcatalyst regeneration at any given time, while the other reactor(s)is/are processing the lower alkane feed under aromatization conditionsto make aromatics. Another example of such a process features afluidized catalyst bed, in which catalyst particles cycle rapidly andcontinuously between a reaction zone where aromatization takes place anda regeneration zone where the accumulated coke is burned off thecatalyst to restore activity. For example, U.S. Pat. No. 5,053,570describes a fluid-bed process for converting lower paraffin mixtures toaromatics.

Requirements for optimal catalyst performance in a process involving arelatively short period of catalyst exposure to reaction conditionsbetween each regeneration treatment, such as a fluidized-bed process,can differ from those of fixed- or moving-bed processes which requirelonger catalyst exposure time to reaction conditions betweenregeneration treatments. Specifically, in processes involving shortcatalyst exposure times, it is important that the catalyst not exhibitexcessive initial cracking or hydrogenolysis activity which couldconvert too much of the feedstock to undesirable, less-valuablebyproducts such as methane.

Certain metals such as Pt which are very suitable for catalyzing thedehydrogenation reactions that are essential for an alkanedehydroaromatization process can also, under certain circumstances,display undesirable hydrogenolysis activity that leads to excessiveproduction of methane from higher hydrocarbons. The inclusion of asecond, inert or less-active metal in a catalyst composition to helpsuppress the hydrogenolysis activity of the first, more-active metal isused in commercial scale catalytic naphtha reforming in which C₅-C₁₂paraffins and naphthenes are converted to aromatic compounds withcatalysts which are predominantly bimetallic and are supported onchloride-promoted alumina. As indicated in a catalytic naphtha reformingreview article by C. A. Querini in volume 6, pages 1-56 of theEncyclopedia of Catalysis (I. T. Horvath, ed.; published by John Wiley &Sons, Inc., Hoboken, N.J., USA, 2003), these catalysts typically containPt plus another metal such as Re (in sulfided form) or Sn. Among othereffects, these second metals can interact with the Pt to reducehydrogenolysis activity, thereby decreasing the rate of unwanted methaneformation.

These Pt/Re and Pt/Sn catalysts, supported on chloride-promoted alumina,are widely employed in fixed-bed (semi-regenerative) and moving-bed(continuous) naphtha reformers, respectively, and their compositions areoptimized for relatively long catalyst exposure times to reactionconditions between regeneration treatments. The average catalystparticle residence time in the reaction zone between regenerationtreatments ranges from a few days in moving bed reactors and up to 1 or2 years in fixed bed reactors. According to the article by Querinimentioned above, typical Pt and Sn levels in Pt/Sn naphtha reformingcatalysts are about 0.3% wt each. Such catalysts, which usually lack astrongly acidic zeolite component, do not work well for lower alkanearomatization.

It would be advantageous to provide a light hydrocarbondehydroaromatization process which can be performed under conditionsthermodynamically favorable for light alkane aromatization as describedabove, which provides for relatively short catalyst exposure time toreaction conditions, wherein the average catalyst particle residencetime in the reaction zone between regeneration treatments may be fromabout 0.1 second to about 30 minutes in a fluidized bed reactor and froma few hours up to a week in moving bed and fixed bed reactors, and inwhich the catalyst composition is optimized to reduce excessive initialproduction of less-desirable byproducts such as methane.

SUMMARY OF THE INVENTION

The present invention provides a process for producing aromatichydrocarbons which comprises:

(a) contacting one or more lower alkanes with a dehydroaromatizationcatalyst wherein the lower alkanes contact time (the average residencetime of a given lower alkane molecule in the reaction zone underreaction conditions) is preferably from about 0.1 seconds to about 1minute, most preferably about 1 to about 5 seconds, preferably at about550 to about 730° C. and about 0.01 to about 1.0 MPa, said catalystcomprising:

-   -   (1) about 0.005 to about 0.1% wt (% by weight) platinum, basis        the metal, preferably about 0.01 to about 0.05% wt,    -   (2) an amount of an attenuating metal selected from the group        consisting of tin, lead, and germanium which is not more than        about 0.2% wt of the catalyst, basis the metal, and wherein the        amount of platinum is not more than about 0.02% wt more than the        amount of the attenuating metal, preferably not more than about        100 ppm more than the amount of the attenuating metal, more        preferably less than or equal to the amount of the attenuating        metal, most preferably from about 50 to about 100 percent of the        amount of the attenuating metal;    -   (3) about 10 to about 99.9% wt of an aluminosilicate, preferably        a zeolite, basis the aluminosilicate, preferably about 30 to        about 99.9% wt, preferably selected from the group consisting of        ZSM-5, ZSM-11, ZSM-12, ZSM-23, or ZSM-35, preferably converted        to the H+ form, preferably having a SiO₂/Al₂O₃ molar ratio of        from about 20:1 to about 80:1, and    -   (4) a binder, preferably selected from silica, alumina and        mixtures thereof;

(b) collecting the products from (a) and separating and recovering C₆₊aromatic hydrocarbons;

(c) optionally recovering methane and hydrogen; and

(d) optionally recycling C₂₋₅ hydrocarbons to (a).

The reactor system may comprise one or more reaction vessels, chambers,or zones, arranged in parallel or in series, in which contact betweenthe catalyst particles and the ethane-containing feed occurs. Thereactor vessel(s), chamber(s), or zone(s) may feature a fixed catalystbed (i.e., with parallel beds), a slowly-moving catalyst bed, or afluidized bed, In a preferred embodiment, a fluidized-bed reactor isused. The process is optimized to minimize the average catalyst particleresidence time while maintaining selectivity and conversion rate. Theaverage catalyst particle residence time is the average amount of timethat a catalyst particle is in the active reaction zone with ethanebetween regenerations.

Catalysts of the present invention—featuring lower levels ofdehydrogenation metal (preferably Pt) with potential cracking function,plus proper moderation of the dehydrogenation metal activity withappropriate amounts of a second, attenuating metal—are designed to limitinitial cracking activity without sacrificing the overall activity andaromatics selectivity required for commercially-viable production ratesof benzene and other aromatics.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for producing aromatic hydrocarbonswhich comprises bringing a hydrocarbon feedstock containing at leastabout 50 percent by weight of ethane or other C₂ hydrocarbons or otherlower alkanes into contact with a dehydroaromatization catalystcomposition suitable for promoting the reaction of lower alkanes toaromatic hydrocarbons such as benzene at a temperature of about 550 toabout 730° C. and a pressure of about 0.01 to about 1.0 MPa. The primarydesired products of the process of this invention are benzene, tolueneand xylene.

The hydrocarbons in the feedstock may be ethane, ethylene or other loweralkanes or mixtures thereof. Preferably, the majority of the feedstockis ethane and from about 0 to about 20 weight percent of the feedstockmay be comprised of ethylene, preferably about 5 to about 10 weightpercent. Lower alkanes may be ethane, propane, butane, pentane, etc. Thefeedstock may contain in addition up to about 40 weight percent of otheropen chain hydrocarbons containing between 3 and 8 carbon atoms ascoreactants. Specific examples of such additional coreactants arepropylene, isobutane, n-butenes and isobutene. The hydrocarbon feedstockpreferably contains at least about 60 percent by weight of ethane orlower alkanes, more preferably at least about 70 percent by weight. Thereaction feed is often referred to herein as ethane for convenience butit is meant to include all of the other hydrocarbon materials referredto above if it is necessary or desired for them to be present.

In a preferred embodiment, the reactor comprises a zone, vessel, orchamber containing catalyst particles through which theethane-containing feed flows and the reaction takes place. The reactorsystem may involve a fixed, moving, or fluidized catalyst bed. Thereaction products then flow out of the bed and are collected. Thereaction products are then separated and C₆₊ aromatic hydrocarbons arerecovered. Optionally, methane and hydrogen are recovered and optionallythe C₂₋₅ hydrocarbons are recycled to step (a).

A fixed bed reactor is a reactor in which the catalyst remainsstationary in the reactor and the catalyst particles are arranged in avessel, generally a vertical cylinder, with the reactants and productspassing through the stationary bed. In a fixed bed reactor the catalystparticles are held in place and do not move with respect to a fixedreference frame. The fixed bed reactor may be an adiabatic single bed, amulti-tube surrounded with heat exchange fluid or an adiabatic multi-bedwith internal heat exchange, among others. Fixed bed reactors are alsoreferred to as packed bed reactors. Fixed bed reactors provide excellentgas solids contacting. The fixed bed reactor configuration may includeat least two separate fixed beds in different zones so that at least onebed can be in active operation under reaction conditions while thecatalyst the other bed(s) is being regenerated.

In a moving bed catalytic reactor, gravity causes the catalyst particlesto flow while maintaining their relative positions to one another. Thebed moves with respect to the wall of the vessel in which it iscontained. The reactants may move through this bed with cocurrent,countercurrent or crossflow. Plug flow is the preferred mode. The movingbed offers the ability to withdraw catalyst particles continuously orintermittently so they can be regenerated outside the reactor andreintroduced into the circuit later on. Thus, there is an advantage tousing a moving bed when the catalyst has a short active life and can becontinuously regenerated. A moving bed reactor may consist of at leastone tray as well as supporting means for one or more catalyst beds. Thesupporting means may be permeable to gas and impermeable to catalystparticles.

A fluidized bed reactor is a type of reactor that may be used to carryout a variety of multiphase chemical reactions. In this type of areactor, a gas is passed through the particulate catalyst at high enoughvelocities to suspend the solid and cause it to behave as though it werea fluid. The catalyst particles may be supported by a porous plate. Thegas may be forced through the porous plate up through the solidmaterial. At lower gas velocities the solids remain in place as the gaspasses through the voids in the material. As the gas velocity isincreased, the reactor reaches the stage where the force of the fluid onthe solids is enough to balance the weight of the solid material andabove this velocity the contents of the reactor bed begin to expand andswirl around much like an agitated tank or boiling pot of water. Afluidized bed reactor is preferred for use in the present inventionbecause it provides uniform particle mixing, uniform temperaturegradients and the ability to operate the reactor in a continuous state.The catalyst leaves the reaction zone with the reaction products and isseparated therefrom in order to be regenerated before being recycled tothe reaction zone.

The ethane contact time may range from about 0.1 second to about 1minute. The ethane contact time is the average amount of time that onemolecule of the ethane feed is in the reaction zone. The preferredethane contact time is from about 1 to about 5 seconds. Longer ethanecontact times are less desirable because they tend to allow forsecondary reactions that lead to less-desirable byproducts such asmethane and reduce selectivity to benzene and/or total aromatics.

The catalyst comprises from about 0.005 to about 0.09% wt platinum,basis the metal. The platinum is highly active in terms of catalyzingthe dehydroaromatization reaction and it is best if its concentration inthe catalyst not be more than 0.1% wt because otherwise too much methanewill be produced. In one embodiment from about 0.01 to about 0.05% wt ofplatinum is used. High performance is thus obtained with relatively lowamounts of metals in the catalyst. The amount of platinum may be notmore than about 0.02% wt more than the amount of the attenuating metal,preferably not more than about 100 ppm more than the amount of theattenuating metal, more preferably less than or equal to the amount ofthe attenuating metal, most preferably from about 50 to about 100percent of the amount of the attenuating metal, basis the metal.

An attenuating metal is an essential component of the catalyst of thepresent invention. The attenuating metal moderates the catalyticactivity of platinum so as to reduce the production of less-valuablemethane byproduct. The attenuating metal may be selected from the groupconsisting of tin, lead, and germanium. The attenuating metal comprisesnot more than about 0.2% wt of the catalyst, basis the metal, morepreferably not more than about 0.15% wt and most preferably not morethan about 0.1% wt of the attenuating metal is utilized because morethan that can cause the overall conversion to aromatics to become toolow for commercial use.

The catalyst also comprises from about 10 to about 99.9% wt of one ormore aluminosilicate materials, preferably from about 30 to about 99.9%wt, basis the aluminosilicate(s). The aluminosilicates preferably have asilicon dioxide:aluminum trioxide molar ratio of from about 20 to about80. The aluminosilicates may preferably be zeolites having the MFI orMEL type structure and may be ZSM-5, ZSM-8, ZSM-11, ZSM-12 or ZSM-35.The zeolite or zeolite mixture is preferably converted to H⁺ form toprovide sufficient acidity to help catalyze the dehydroaromatizationreaction. This can be accomplished by calcining the ammonium form of thezeolite in air at a temperature of at least about 400° C.

The binder material serves the purpose of holding invidivual zeolitecrystal particles together to maintain an overall catalyst particle sizein the optimal range for fluidized-bed operation or to prevent excessivepressure drop in fixed or moving bed operation. The binder may beselected from the group consisting of alumina, silica, silica/alumina,various clay materials such as kaolin, or mixtures thereof. Preferably,amorphous inorganic oxides of gamma alumina, silica, silica/alumina or amixture thereof may be included. Most preferably, alumina and/or silicaare used as the binder material.

A platinum containing crystalline aluminosilicate, such as ZSM-5, may besynthesized by preparing the aluminosilicate containing the aluminum andsilicon in the framework, depositing platinum on the aluminosilicate andthen calcining the aluminosilicate. The attenuating metal may also beadded by the same procedure, either prior to, simultaneously with, orafter the addition of platinum. The metals may be added by any commonlyknown method for adding metals to such structures includingincorporation into the aluminosilicate framework during crystalsynthesis, or subsequent ion exchange into an already-synthesizedcrystal framework, or well as by various impregnation methods known tothose skilled in the art. The platinum and attenuating metal may beadded by the same or different methods.

In a preferred embodiment of the present invention an ethane or loweralkane feedstream is introduced into the dehydroaromatization reactor.The feedstream then comes into contact with the catalyst particles forthe prescribed period of time. The reaction products leave the reactorand are transferred into a separator. The separator removes the aromaticproducts and the principal byproducts, methane and hydrogen, whichpreferably may be recovered, and also removes C₂₋₅ byproducts andunreacted ethane or other lower alkanes which optionally may be recycledto the dehydroaromatization reactor.

EXAMPLES

The following examples are illustrative only and are not intended tolimit the scope of the invention.

Example 1

Catalysts A through I were prepared on samples of an extrudate materialcontaining 80% wt of CBV 3014E ZSM-5 zeolite (30:1 molar SiO₂:Al₂O₃ratio, available from Zeolyst International) and 20% wt of aluminabinder. The extrudate samples were calcined in air up to 650° C. toremove residual moisture prior to use in catalyst preparation.

Metals were deposited on 25-50 gram samples of the above ZSM-5/aluminaextrudate by first combining appropriate amounts of stock solutions oftetraammine platinum nitrate and tin(IV) tetrachloride pentahydrate,diluting this mixture with deionized water to a volume just sufficientto fill the pores of the extrudate, and impregnating the extrudate withthis solution at room temperature and atmospheric pressure. Impregnatedsamples were aged at room temperature for 2-3 hours and then driedovernight at 100° C.

To determine the platinum and tin contents of the catalysts, a sample ofthe catalyst was calcined at 550° C. to drive off residual moisture torender a loss on ignition (LOI) percentage. A known mass of theuntreated ground catalyst, corrected by LOI percentage, was digestedusing closed vessel microwave acid digestion involving nitric,hydrochloric, and hydrofluoric acids. The solution was diluted to aknown volume with deionized water and then analyzed for the indicatedmetals by directly coupled plasma emission analysis. Results arereported as ppmw or weight percent based on the weight of the 550°C.-calcined catalyst sample.

Catalysts made on the ZSM-5/alumina extrudate were tested “as is,”without crushing. For each performance test, a 15-cc charge of catalystwas loaded into a quartz tube (1.40 cm i.d.) and positioned in athree-zone furnace connected to an automated gas flow system.

Prior to performance testing, all catalyst charges were pretreated insitu at atmospheric pressure as follows:

(a) calcination with air at 60 liters per hour (L/hr), during which thereactor wall temperature was increased from 25 to 510° C. in 12 hrs,held at 510° C. for 4-8 hrs, then further increased from 510 to 630° C.in 1 hr, then held at 630° C. for 30 min;

(b) nitrogen purge at 60 L/hr, 630° C. for 20 min;

(c) reduction with hydrogen at 60 L/hr, 630° C. for 30 min

At the end of the pretreatment, 100% ethane feed was introduced at 15L/hr (1000 gas hourly space velocity-GHSV), atmospheric pressure, withthe reactor wall temperature maintained at 630° C. The total reactoroutlet stream was sampled and analyzed by an online gas chromatographysystem two minutes after ethane feed addition. Based on composition dataobtained from the gas chromatographic analysis, initial ethaneconversion and hydrocarbon product selectivities were computed accordingto the following formulas:

ethane conversion, %=100×(100−% wt ethane in outlet stream)/(% wt ethanein feed)

selectivity to hydrocarbon product Y (other than ethane)=100×(moles ofcarbon in amount of product Y generated)/(moles of carbon in amount ofethane reacted)

For purposes of the selectivity calculation, C₉₊ aromatics were assumedto have an average molecular formula of C₁₀H₈ (naphthalene).

Analyzed platinum and tin levels and initial aromatization performancedata for Catalysts A-I, prepared and tested as described above, arepresented in Table 1. The data in Table 1 indicate that thelow-Pt/Sn/ZSM-5 catalysts A through E of the present invention (lessthan 0.1% wt Pt, Sn level such that the amount of platinum is no morethan 0.02% wt more than the amount of Sn, not above 0.1% wt of thecatalyst) provide better initial suppression of methane production andhigher selectivity to benzene and total aromatics under ethanearomatization conditions than the catalysts F through I, in which thePt/Sn levels fall outside the ranges of the present invention.

Example 2

Catalysts J through N, containing various levels of Pt and Ge, wereprepared, analyzed, and tested as described in Example 1 above, exceptthat appropriate amounts of germanium(IV) oxide, dissolved in a diluteaqueous ammonium hydroxide solution, were used instead of tin(IV)chloride pentahydrate. Analyzed platinum and germanium levels andinitial ethane aromatization performance data obtained with thesecatalysts by the test protocol described for Example 1 are presented inTable 2. The data in Table 2 indicate that the low-Pt/Ge/ZSM-5 catalystsK through M of the present invention provide better initial suppressionof methane production and higher selectivity to benzene and totalaromatics than catalysts J and N, in which the Pt/Ge levels fall outsidethe ranges specified in the present invention.

TABLE 1 Catalyst A B C D E F G H I Analyzed Pt Level, % wt 0.006 0.0110.025 0.0437 0.040 0.100 0.103 0.123 0.233 Analyzed Sn Level, % wt 0.0050.010 0.012 0.0395 0.093 0.076 0.0601 0.110 0.217 Ethane conversion, %44.4 44.72 48.02 50.39 45.42 55.44 61.62 55.73 56.6 Selectivities, %(carbon basis) Methane 15.68 18.84 24.22 24.64 21.1 30.16 38.39 29.8529.67 Ethylene 13.86 14.18 12.6 11.17 9.84 10.37 8.88 9.46 9.3 Propylene2.23 2.13 1.63 1.36 1.27 1.1 0.85 0.97 0.86 Propane 1.67 1.69 1.4 1.231.56 0.91 0.64 0.84 0.76 C4 Hydrocarbons 0.46 0.43 0.33 0.28 0.28 0.240.18 0.21 0.19 C5 Hydrocarbons 0.04 0.04 0.01 0 0.01 0 0.01 0.01 0.04Benzene 35.19 37.18 36.61 34.32 36.54 31.67 30.39 31.59 31.69 Toluene18.48 19.28 18.27 18.05 19.17 16.34 14.85 16.53 15.94 C8 Aromatics 3.733.83 3.27 3.7 3.82 2.9 2.57 3.3 3 C9+ Aromatics 8.68 2.4 1.65 5.24 6.416.31 3.24 7.24 8.56 Total Aromatics 66.07 62.69 59.81 61.31 65.94 57.2251.05 58.66 59.19

TABLE 2 Catalyst J K L M N Analyzed Pt Level, 0.0460 0.0436 0.04410.0436 0.1220 % wt Analyzed Ge Level, 0.0216 0.0442 0.0844 0.1210 0.1235% wt Ethane conversion, % 46.94 46.39 46.6 45.07 50.16 Selectivities, %(carbon basis) Methane 22.65 18.24 16.27 15.36 20.81 Ethylene 9.51 11.9712.67 12.96 11.33 Propylene 1.19 1.5 1.54 1.65 1.2 Propane 1.44 1.481.47 1.62 1.15 C4 Hydrocarbons 0.27 0.32 0.34 0.36 0.29 C5 Hydrocarbons0.03 0 0.04 0.02 0.03 Benzene 35.52 37.4 37.28 36.68 35.43 Toluene 19.2519.8 19.85 19.73 18.35 C8 Aromatics 4.07 3.68 4.15 3.95 3.7 C9+Aromatics 6.06 5.6 6.4 7.67 7.71 Total Aromatics 64.91 66.48 67.68 68.0365.2

1. A catalyst comprising: (a) about 0.005 to about 0.1% wt platinum,basis the metal, (b) an amount of an attenuating metal selected from thegroup consisting of tin, lead, and germanium which is not more thanabout 0.2% wt of the catalyst, basis the metal, and wherein the amountof platinum is not more than about 0.02% wt more than the amount of theattenuating metal, (c) about 10 to about 99.9% wt of an aluminosilicate,and (d) a binder.
 2. The catalyst of claim 1 wherein the amount ofplatinum is from about 0.01 to about 0.05% wt.
 3. The catalyst of claim1 wherein the catalyst comprises not more than about 0.15% wt of theattenuating metal.
 4. The catalyst of claim 1 wherein the amount of thealuminosilicate is from about 30 to about 99.9% wt.
 5. The catalyst ofclaim 1 wherein the aluminosilicate has a silicon dioxide:aluminumtrioxide molar ratio of from about 20 to about
 80. 6. The catalyst ofclaim 1 wherein the aluminosilicate is a zeolite.